1. Field of the Invention
The invention relates to connectors used to join the ends of pipes together and, more particularly, to expansion joints which permit the ends of joined pipes to move relative to each other.
2. Description of the Prior Art
Expansion joints, generally, are flexible connectors of pipe ends. They usually take the form of a tube composed of reinforced rubber or other elastomer having integral, radially extending flanges at each end. One or more expandible annular arches integral with the tube may be disposed intermediate the flanges. The flanges may have apertures about their periphery through which bolts may be fitted in order to secure the expansion joint flanges to flanges of pipes which are desired to be flexibly connected.
As a means for connecting pipes together, expansion joints offer numerous benefits. Expansion joints are lightweight and small as compared with metal expansion bends or loops. Thus, installation labor is minimized and, when bolted into place, the expansion joint takes up a minimum of space. More importantly, expansion joints permit the controlled axial movement of the joined pipes by compressing or elongating while at the same time compensating for lateral, torsional, and angular pipe movement. Vibration and sound transmission are also considerably reduced using an elastomeric expansion joint as compared to metal expansion bends or loops.
In corrosive applications, elastomer expansion joints may be lined with a fluorocarbon polymer tube, such as tetrafluoroethylene (TFE). Fluorocarbon polymers are characteristically resistant to chemical attack, are heat stable, have a non-stick surface, and a low coefficient of friction. While expansion joints having fluorocarbon polymer liners offer the above advantages, there are several problems associated with the use of such liners which have not been adequately addressed.
One of these problems relates to the non-stick characteristic of fluorocarbon polymers. When a fluorocarbon polymer liner is used, it is desirable to have it firmly secured to the rubber of the expansion joint, especially in vacuum applications where the liner might otherwise pull away. But being non-stick, it is difficult to secure a fluorocarbon liner to the rubber cover of an expansion joint. A solution to this adhesion problem has been to first embed a knit cloth into the fluorocarbon polymer liner as taught by MacDonald, U.S. Pat. No. 3,723,234, the disclosure of which is incorporated herein by reference. The cloth thus is mechanically interlocked with the fluorocarbon liner, providing a glueable surface by which the otherwise non-stick liner can be adhered to the rubber. Even so, due to the very nature and purpose of expansion joints, the rubber back tends to flex, an expected and desirable feature. However, the flexing of the rubber back may also result in a separation of the rubber back from the liner, especially in the vicinity of the expansion joint flanges where the seal with the pipes is effected. Further, when expansion joints are used in a corrosive environment, corrosives tend to seep in between the fluorocarbon polymer liner and the rubber back at the flange, deteriorating the rubber and attacking the adhesive bond, causing separation. Once the rubber back separates from the liner, the integrity of the expansion joint is lost and the rubber back no longer provides structural support for the liner. The end result is a leakage of corrosives at the site of the pipe-expansion joint seal, resulting in a pressure drop in the line and plant down-time.
Part of the leaking problem originates with the integral rubber or elastomer flanges at either end of the expansion joint. These flanges generally have a large radial dimension and include a plurality of apertures through which bolts may be fitted. The flanges are bolted between a back-up ring and the pipe flange; a gasket usually is interposed between the expansion joint flange and the pipe flange. But unlike the rigid pipe flange and back-up ring, the expansion joint flange is flexible. Thus, when the pipes to which the expansion joint is bolted begin to move about, the flexible expansion joint will distort somewhat, resulting in only a partial seal at the expansion joint flange-gasket-pipe flange interface. Also, a partial seal may result at the places where the bolts pass through the apertures. Leakage of whatever substance is being piped then will result. Further, this leakage will aggravate any separation which might have begun already between the liner and rubber back as a result of flexing.
Yet an additional concern relates to the apertures formed in the flanges. Since the flanges are rigidly connected to the body of the expansion joint, the apertures cannot be moved circumferentially about the expansion joint except by applying torsional forces to the flanges. In turn, misalignments between apertures in pipe flanges and the apertures in the expansion joint flanges will be difficult to correct. It is possible that the expansion joint will have to be connected between pipe ends in a twisted condition, thus aggravating problems relating to leakage at the flange-gasket-pipe flange interface.
Finally, where a lined expansion joint is used under vacuum conditions and elevated temperatures, the liner has a tendency to pull away from the rubber back due to the poor adhesion characteristics of fluorocarbon polymers. While this problem is alleviated somewhat by the knit cloth embedded in the liner as previously discussed, with the additional forces imposed on the liner by a vacuum, coupled with the distortion of the expansion joint due to pipe movement and the bond-destroying effects of chemical attack, especially acute at elevated temperatures, it will be only a short period of time before the liner pulls away from the rubber back, destroying the integrity and usefulness of the expansion joint.